1. Field of the Invention
The present invention relates to an organic light-emitting device.
2. Description of the Related Art
Investigations have been vigorously conducted on organic light-emitting devices as flat light sources and as components for thin display panels at present. The organic light-emitting devices are each an electron device formed of: two electrodes; and a multilayer thin film formed of organic amorphous charge transport materials interposed between the two electrodes. In addition, the organic light-emitting devices are each such an electron device that charges (holes and electrons) are injected from the electrodes toward the thin film, the charges recombine in a specific compound to excite the specific compound, and light emitted in the deactivation process of the excited compound is extracted. Reports on the organic light-emitting devices available since olden times are, for example, light emission by the application of a voltage to an anthracene single crystal and light emission by a two-layer thin film of organic amorphous charge transport materials by Tang, et. al. In addition, in recent years, organic light-emitting devices and the like whose multilayer thin films are each formed of a double heterostructure (three-layer structure) formed of a hole transport layer, an emission layer, and an electron transport layer have also been proposed.
Meanwhile, fluorescent light-emitting materials each of which extracts a singlet exciton component out of the produced excitons as light emission and phosphorescent light-emitting materials each capable of extracting each of both a singlet exciton component and a triplet exciton component out of the produced excitons as light emission have been proposed as light-emitting materials in the organic light-emitting devices. By the way, excitons produced by charge recombination may be produced at a ratio “singlet exciton:triplet exciton” of 1:3 based on a spin selection rule. Here, each of the phosphorescent light-emitting materials can extract each of both a triplet exciton component and a singlet exciton component to be produced as light emission. Therefore, the phosphorescent light-emitting materials are extremely promising light-emitting materials in high-efficiency organic light-emitting devices because the materials each have an internal quantum efficiency of 100% in theory.
In view of the foregoing, the efficiency of each of organic light-emitting devices using the phosphorescent light-emitting materials can be improved as compared with that of each of the conventional organic light-emitting devices using the fluorescent light-emitting materials in theory. However, luminous efficiency sufficient for practical use has not been realized at present. For example, the external quantum yield of an organic light-emitting device formed of a combination of CBP that has been widely used as a host for a phosphorescent light-emitting material up to now and a phosphorescent light-emitting material (Btp)2Ir(acac) is around 7% at the maximum (Physical Review B, Volume 62, Number 16, p10967 (2000)). In other words, an improvement in efficiency, and a reduction in voltage, of each of the organic light-emitting devices using the phosphorescent light-emitting materials are insufficient at present, and hence an organic light-emitting device with its performance additionally improved from the viewpoints of efficiency and a voltage has been desired.
In addition, at present, each of the organic light-emitting devices using the phosphorescent light-emitting materials shows high luminous efficiency in a low-current state in which an emission luminance is low but undergoes a remarkable reduction in luminous efficiency in a high-current state in which the emission luminance is high. This is because, in the case of the phosphorescent light-emitting devices, the deactivation process of triplet excitons called triplet-triplet annihilation (T-T annihilation) is present in the high-current state (Physical Review B, Volume 62, Number 16, p10967 (2000)). The T-T annihilation occurs at a current density of about 1 mA/cm2 in, for example, a platinum complex having a long phosphorescence lifetime. In addition, the T-T annihilation occurs at a current density of 10 mA/cm2 to 100 mA/cm2 in, for example, an iridium complex having a short phosphorescence lifetime. The T-T annihilation is a cause for the lowering of the emission luminance requested of each of the organic light-emitting devices as components for displays and the like, though the extent of the lowering varies depending on luminous efficiency.
Meanwhile, in the case of the organic light-emitting devices using the phosphorescent light-emitting materials, the lifetime of phosphorescence emitted from each of the light-emitting materials is long. As a result, a ratio of the amount of triplet excitons each of which is deactivated (emits light) to the amount of excitons produced per unit time increases, and hence the luminous efficiency of each of the devices reduces. This is because, in a high-current state, the density of the excitons in an emission layer increases, the triplet excitons are apt to collide with each other, and the probability that radiationless deactivation occurs increases. Accordingly, each of the organic light-emitting devices utilizing phosphorescence has involved the following problem. Each of the devices has high luminous efficiency in a low-luminance region but cannot utilize the high luminous efficiency in a high-luminance region, and as a result, its power consumption increases.
Meanwhile, Japanese Patent Application Laid-Open No. 2003-68461 discloses a technology involving incorporating a host, a phosphorescent light-emitting material, and a current promoter into an emission layer to enable low-voltage driving while injecting an additionally large amount of carriers into the emission layer at a low voltage. Here, in Japanese Patent Application Laid-Open No. 2003-68461, the constituents of the emission layer are selected so as to compensate the charge transport properties of the host and the current promoter, respectively. For example, the document discloses the following combination. When the host is electron transportable, the current promoter is hole transportable.
On the other hand, Japanese Patent Application Laid-Open No. 2006-128632 discloses, as a method of improving luminous efficiency, a technology involving using a fluorene multimer as a host and adding a second dopant for causing the intersystem crossing of excitons as well as a phosphorescent light-emitting dopant to an emission layer. The technology enables each of both a singlet exciton and a triplet exciton produced on the host to efficiently transfer its own excitation energy to the light-emitting dopant, and hence a considerable improvement in efficiency has been realized as compared with a conventional one.
Meanwhile, a technology by which high-quality display can be realized with an additionally low power consumption has been requested in association with the progress of a display technology in recent years. For example, a method involving representing an image to be displayed while partially increasing its luminance when the variations of light and shade exist in the image and a method involving providing a high-luminance display mode to correspond to outdoor display are available. On the other hand, time-division driving, passive driving, and the like have also been proposed as methods of driving devices, and light-emitting properties at high luminances have become more important than ever before.
Here, the method of Japanese Patent Application Laid-Open No. 2003-68461 described above may result in the formation of a charge transfer complex or exciplex when an electron transportable compound having electron-accepting property and a hole transportable compound having electron-donating property strongly interact with each other because both the compounds are mixed in the same layer. In addition, excitons produced by the recombination of holes and electrons disappear owing to the formation of the charge transfer complex or exciplex, and as a result, luminous efficiency may reduce. In addition, an additional investigation on the case where a high current is flowed is needed because a carrier balance may be lost and the luminous efficiency may reduce in the case.
In addition, the method of Japanese Patent Application Laid-Open No. 2006-128632 has been unable to crack a challenge of suppressing a reduction in luminous efficiency in a high-current region, and hence an additional investigation is needed again. In addition, when a phosphorescent light-emitting dopant whose luminescent color is green or blue is used, the number of kinds of materials capable of being responsible for intersystem crossing each of which is to be used as the second dopant is small, and hence it has been difficult to optimize a combination of the dopants.